The purpose of the proposed research is (a) to apply a quantitative psychological theory of timing to a standard psychophysical reference experiment for timing, (b) to evaluate the role of various neurotransmitters in timing performance, and (c) to relate the neurobiological variables to the psychological variables. Experimental studies will be conducted with rats in a temporal estimation procedure. A model of scalar timing will be adapted to provide a quantitative fit of experimental results. The parameters of the theory can be identified with specific psychological variables, such as "attention," "responsiveness," "sensitivity to time," "clock speed," and "memory of reinforced time." Studies with methamphetamine, haloperidol, and related drugs will be conducted to evaluate the hypothesis that the speed of the internal clock is related to the level of dopamine. Other studies will be conducted to determine what neurotransmitter mechanism might be responsible for the improved timing performance following administration of vasopressin. The long-range objective is to determine the role of neurotransmitters in higher cognitive processes, such as timing. The scientific disciplines are experimental psychology and psychobiology.